In LTE Licensed-Assisted Access (LAA), an LTE carrier operating in unlicensed spectrum is aggregated with an LTE carrier in licensed spectrum. The subframe boundaries on the unlicensed carrier would typically be aligned with those on the licensed carrier.
Before data transmission can take place on the unlicensed carrier, regulations in some parts of the world require that a Clear Channel Assessment (CCA) or Extended CCA (ECCA) procedure is carried out to sense the channel to determine that it is free of transmissions from other devices. If the (E)CCA carried out by a device determines that the channel is free, the device may commence transmission immediately.
Further regulations in some parts of the world impose a restriction on the maximum length of transmission time by a given device (e.g. 4 ms). Currently in LTE, data transmissions always terminate at a subframe boundary, since there is no mechanism to indicate any other termination point. However, with subframes having a duration of 1 ms, up to 25% of the available transmission time may not be utilized in each transmission opportunity if data transmissions have to terminate at the last subframe boundary before 4 ms after the start of transmission.
A mechanism is therefore needed to enable data transmissions to terminate part way through a subframe.